Efforts to reduce energy consumption and toxic emissions in the application and curing of coatings have led coatings producers to develop so called "100 percent solids" radiation curable coatings. These coatings comprise mixtures of reactive compounds which can be applied to substrates as liquids at room temperature and which polymerize upon exposure to radiation to produce solid coatings. Since all the coating substituents are reactive, the need to apply heat to evaporate solvents is eliminated. Numerous coatings of this type have been disclosed in recent years. However, it is always desirable that new compositions be developed which satisfy the physical requirements of particular coating applications.
In some instances, it is desired to apply a 100 percent solids radiation curable coating as a protective layer over printing or a design on a sheet of metal which will subsequently undergo a metal forming operation. An example of such an application is a clear coating applied as a protective layer over the printing or design on a sheet of metal which is subsequently formed into a bottle cap. To be useful in such an application the cured coating must be hard enough to afford protection for the printing or design, yet flexible enough to bend during the metal forming operation without cracking or crazing. Moreover, it is desirable that the viscosity of the uncured coating composition be such that it can be applied to the substrate by conventional film application methods such as reverse roll coating or direct roll coating.
Frequently, in handling the coated metal sheets or coated formed articles on production lines, it is desired to slide a coated sheet or formed article across a metal surface or a similarly coated surface. For example, in stacking and unstacking operations one may desire to slide the coated or uncoated side of one coated sheet of metal across the coated side of another sheet of metal. In such instances, it is necessary that the coated surface have a sufficient degree of surface lubricity to prevent sticking or damage to the coating due to abrasion. If the coating does not have sufficient inherent surface lubricity, one can impart the desired lubricity by applying to the coating surface a film of lubricant, such as a lubricating oil. However, this is inconvenient since it necessitates additional processing steps and machinery for the application of the lubricant and for its removal when it is no longer needed. Further, lubricating oils can cause certain types of radiation curable coatings to become hazy in appearance.
It is an object of the present invention to provide radiation curable coating compositions which are sufficiently hard and flexible in the cured state to be used as a protective coating over printing or designs on a sheet of metal which is to undergo metal forming operations.
It is a further object of this invention to provide radiation curable compositions, as aforesaid, which have sufficient inherent surface lubricity in the cured state to allow the coated surface to slide easily across metal surfaces or similarly coated surfaces during processing of the coated metal. Other objects and advantages of this invention will be apparent to those skilled in the art from the disclosure herein.